This is a double-blind, placebo controlled clinical trial of oral melatonin administration in sighted subjects between the ages of 21 and 65. The specific goal of this project is to characterize the shape of the melatonin phase response curve (PRC) in subjects age 21 to 65. Other study objectives are: 1) to characterize the shape of the melatonin PRC, 2) to assess the variability of the shape of the PRC, and 3) to develop melatonin as a treatment for circadian phase disorders.